Acute liver failure (ALF) is a devastating condition that affects patients with and without prior liver disease, including the veteran patients served by the Veterans Administration. Although the causal factors for ALF are generally known, there is a lack of effective pharmacologic treatment as the intra- and intercellular mechanisms of ALF are poorly understood. Kupffer cells (KCs), neutrophils, monocytes, and T cell-mediated immune reactions have been implicated as critical components of the pathogenesis of various types of liver damage. Yet, the morbidity and mortality due to acute liver injury of various etiologies continue to be high (>20,000 deaths/year in the USA) with liver transplantation as the only life-saving option. However, organ shortage, increasing costs, as well as associated complications that cause delay, or even rapid failure, of the graft function are major limitations. Thus despite extensive research, there are as yet unknown mechanisms responsible for ALF, which must be identified. Our work established that perisinusoidal hepatic stellate cells (HSCs) play an important role in endotoxin- as well as ischemia/reperfusion-induced liver injury by producing cytokines and chemokines and by recruiting neutrophils. Our new results show that depletion of HSCs imparts almost complete protection against concanavalin A (ConA)-induced liver injury, a model that has been consistently used to investigate immune cell-mediated liver damage in vivo. Even more important, we found that ConA- stimulated HSCs elicit a direct effect on hepatocytes, illustrated by high oxidative stress and death by apoptosis and necrosis Based on these findings, we hypothesize that HSCs play a central role in various types of acute liver injury from the initial stages. We propose to use the ConA-challenged HSC-depleted mouse and complementary in vitro cell culture systems as models to identify the mechanisms of ALF in 3 Specific Aims- Aim 1: To determine the role of HSCs in ConA-induced acute liver injury. Aim 2: To define the mechanisms of the direct effect of ConA on HSCs and then hepatocytes. Aim 3: To dissect the mechanisms regulating interactions between ConA-stimulated HSCs and cells of the immune system. We contend that understanding of the precise mechanisms by which HSCs orchestrate excessive liver damage directly via soluble mediators and indirectly by interacting with cells of the immune system including KCs is very critical. Such knowledge will be highly significant for the development of new therapies to target HSCs or factor(s) thereof in order to arrest the ongoing hepatic damage responsible for morbidity and mortality of ALF and to prevent a need for transplantation that would otherwise be necessary.